The present invention relates to printed circuit wiring boards composed of a polynorbornene substrate laminated to a copper foil using silane coupling agents.
The purpose of the present invention is to produce printed circuit wire boards which have a low dielectric constant and high bonding strength.
Such laminates are generally compared in the market place for dielectric constant, dissipation factor, chemical resistance, peel strength, solder bath resistance (at 260.degree. C. to 288.degree. C.), warping and punchability.
In conventional processes, so called "prepregs" are made by dipping a pretreated fibrous substrate (fiberglass) in Epoxy or some other solution of polymer resin having good strength and electrical insulating properties and drying the fibrous substrate to remove the solvent and provide a resin-impregnated substrate. It is known to treat the glass substrate with a silane compound to promote the adhesion between the substrate and the resin.
Cellulosic and fiberglass woven materials have long been used to reinforce polymer substrates. It is known silane coupling agents can be applied directly to glass filaments to improve the flexural strength of glass cloth laminates of a variety of resins, often by as much as 300 percent for compression molded test samples. Silane coupling agents at the interface allow many particulate minerals to become reinforcing fillers in composites to increase strength, hardness, modulus, heat distortion and impact strength. Fiberglass cloth is usually treated with an aqueous coupling agent.
Two or more of these prepregs are then pressed together to form an insulating layer for a printed circuit wiring board. To provide the conducting layer for the laminate, one or more copper layers are generally pressed against the exposed surfaces of these prepregs.
Other methods of applying metals to these insulating layers or substrates are vapor deposition, electroplating, sputtering, ion plating, spraying and layering. The metals commonly used are copper, nickel, tin, silver solder, gold, aluminum, platinum, titanium, zinc and chrome, with copper being used most often in printed wire boards.
A problem associated with forming thin metallic coatings on insulating layers or substrates has been the inability to form a complete bond having excellent bond strength between the metallic layer and the substrate and subsequently good solder resistance.
Silane compounds have found wide acceptability for improving adhesion between different substrates.
Silane coupling agents modify the interface between metal or mineral surfaces and organic resins to improve adhesion between the surface and the resin. The physical properties and water resistance of the reinforced resins are thereby improved. It is believed that silane coupling agents for bonds with metal surfaces through the silane functional group. The hydrolyzed silanes will condense to oligomeric siloxanols and eventually to rigid cross-linked structures. Contact with a polymer matrix should take place while the siloxanols still have some solubility. Bonding to a polymer matrix may take different forms or a combination of forms. Bonding may be covalent where the oligomeric siloxaol is compatible with the liquid matrix resin. The solutions might also form an interpenetrating polymer network as the siloxanols and the resin separately cure with only limited copolymerization.
It is well known that not all silanes or mixtures of silanes will bond all metals to all substrates. McGee, U.S. Pat. No. 4,315,970, states that
"[i]t is generally accepted that specific silanes can be used for adhesion of specific materials to specific substrates. That is, the silane must be matched to the application and it cannot be assumed that all silanes will work in all applications."
Therefore, the suitability of a silane bonding agent in improving adhesion of a metal to a substrate is unpredictable and it must be determined by experimentation.
While suitable coupling agents are commercially available for bonding of many common plastics with a variety of metals, the application of silane coupling agents for bonding of polynorbornenes to metals is not believed by applicant to be previously known.
Norbornene type monomers are polymerized by either a ring-opening mechanism or by an addition reaction wherein the cyclic ring structure remains intact. Ring-opening polymerizations are discussed with greater particularity in U.S. Pat. Nos. 4,136,247 and 4,178,424, assigned to the same assignee as the present invention and are incorporated herein by reference for their discussion of such polymerizations. Ring-opening polymerization generally yields unsaturated linear polymers while addition polymerizaton yields polycycloaliphatics. It is desirable to produce polymers having high molecular weight monomers incorporated therein to provide good temperature resistance, i.e., high heat distortion temperatures and high glass transition temperatures.